My Textured paint became plain color when I render in Cycles
the texture needs to be assigned to a material on this object to be visible on the render , and to start using the nodes editor :


select the object
in the material tab click New button to create new material
click use nodes 





in the node editor press  Shift+A➝Texture➝Image texture
in the texture node select you image ( the one you painted )
connect the texture node to the shader node ( click and drag the dot to the other one )